Correlated color temperature

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CIE (1931) xy chromaticity diagram including the Planckian locus, with temperatures indicated. Wavelengths of monochromatic light are shown in blue. The lines crossing the Planckian locus are lines of constant correlated color temperature. The Kelvin system for lamp description works well for an incandescent lamp. Since there are very nearly black body radiators, their chromaticity coordinates land directly on the Plnckian locus in the CIExy color space. Fluorescent lighting is not incandescent and presents a new challenge. Fluorescent lamps are made using myriad combinations of phosphors and gases. The illumination that they produce is almost never described by a point in color space that lies on the Planckian locus. The question then becomes how to describe the quality of light from a fluorescent lamp. The method used is called the “non-elated color temperature”, which is a method for assigning a color temperature to a color near, but not on, the Planckian locus. The above plot shows lines crossing the Planckian locus for which the correlated color temperature is the same. Nevertheless, the colors are not the same, and the method gives only an approximate specification of a particular color. Due to this shortcoming, the rated CCT of any fluorescent tube does not completely specify its color. To be more precise: a number of color spaces have been developed in which the distance between them on a chromaticity diagram may estimate the difference between two colors. There include the 1960 CIELuv (which is now outdated) and the 1976 CIELuv’v’ and CIELab spaces. On a chromaticity diagram for which distances specify color distances, the best estimate of the color temperature of any point will be the color temperature of the point on the Planckian locus closest to that point. Although it is outdated, the CIE specifies distances in the 1960 CIELuv chromaticity space to define correlated color temperature. Photographers often use color temperature meters. Color temperature meters are designed to read only two regions along the visible spectrum (red & blue), more expensive ones read three regions (red, green & blue). They are almost useless under fluorescent light. There are general guidelines and some specific filters recommended to obtain optimum quality under such frustrating circumstances.